Conjugating a water-soluble polymer such as poly(ethylene glycol) (or “PEG”) to a biologically active agent results in a polymer-active agent conjugate that often has advantageous properties over the corresponding “unconjugated” version of the active agent. Among other advantages, conjugated forms of active agents have increased half-lives and are less immunogenic. When PEG is used to form a polymer-active agent conjugate, the conjugated active agent is conventionally referred to as “PEGylated.” Commercially available PEGylated preparations include PEGASYS® PEGylated interferon alpha-2a (Hoffmann-La Roche, Nutley, N.J.), PEG-INTRON® PEGylated interferon alpha-2b (Schering Corp., Kennilworth, N.J.), NEULASTA™ PEG-filgrastim (Amgen Inc., Thousand Oaks, Calif.) and SOMAVERT® pegvisomant (Pfizer, New York, N.Y.). The commercial success of these preparations attests to the value of PEGylation technology.
Polymers bearing a terminal carboxylic acid are useful, either directly or indirectly, in conjugation reactions with active agents and other substances. For example, carboxylic acids can be reacted directly with an amino or hydroxyl group of an active agent, thereby forming a conjugate. Indirectly, polymers bearing a terminal carboxylic acid (which acts as a reactive electrophilic group) can serve as a convenient starting material for preparing other polymer derivatives bearing functional groups other than carboxylic acids. Polymers bearing a functional group other than a carboxylic acid can then form conjugates with active agents bearing a suitable reactive group.
Methods for preparing certain water-soluble polymers bearing a terminal carboxylic acid have been described. For example U.S. Pat. No. 5,681,567 describes reacting a poly(alkylene oxide) with a tertiary-alkyl haloacetate to thereby form a tertiary-alkyl ester of a poly(alkylene oxide) carboxylic acid. Schematically, the reaction using a tertiary-alkyl chloroacetate can be represented as follows:
wherein each R is alkyl. Subsequent reaction of the ester with an acid removes the tertiary alkyl moiety, which yields the corresponding acetic acid. This method, however, only results in polymers bearing a terminal acetic acid moiety. Polymer derivatives synthesized to terminate in an acetic acid moiety are sometimes referred to as “carboxymethylated” polymers.
Polymer derivatives bearing a terminal acetic acid can be further reacted to form polymer derivates bearing other reactive moieties. For example, a succinimidyl ester of carboxymethyl PEG can be formed. This succinimidyl ester, however, is so reactive that it hydrolyzes almost immediately in aqueous solution. Thus, the practical utility of PEG derivatives bearing a terminal acetic acid moiety can be low given the overly reactive nature of these derivatives.
Another method for preparing certain water-soluble polymers bearing a terminal carboxylic acid derivative is described in U.S. Pat. No. 5,523,479. In this approach, a moiety having a molecular weight of from 32 to 6000 and having from one to 6 hydroxyl groups is reacted with a tertiary alkyl ester of a beta-unsaturated carboxylic acid to yield a product having a terminal ester. Schematically, the reaction in can be represented as follows (the moiety is presented as having a single hydroxyl group and the tertiary alkyl ester of a beta-unsaturated carboxylic acid is represented by tertiary alkyl ester of acrylic acid)
wherein R is alkyl. A subsequent hydrolytic step transforms the ester into the corresponding propanoic acid.
While providing polymer derivatives that lack a reactive acetic acid moiety, this method suffers from other drawbacks. First, the method inherently provides for only propanoic acid derivates. In addition, the best reported conversion of the hydroxyl group to the ester is less than 85%. Finally, only moieties having a molecular weight between 32 and 6000 are described in connection with carrying out the method. There remains a need, however, to provide a method that can prepare acids other than propanoic acid derivatives, result in conversion to an ester and/or acid of greater than 85%, and be used with moieties having a molecular weight outside of the range of 32 to 6000.
U.S. Pat. No. 5,672,662, discloses PEG derivatives having a terminal propanoic acid or butanoic acid moiety that can be used to prepare active esters suitable for conjugation to proteins or other molecules bearing amino groups. The active esters described in U.S. Pat. No. 5,672,662 exhibit greater stability in solution than active esters of carboxymethylated PEG, and are thus better suited for conjugation to biologically active molecules. The method described for preparing these PEG derivatives having a terminal propanoic or butanoic moiety, however, involves numerous steps and only results in about 80% substitution into the carboxylic acid moiety. As a consequence, the method described in U.S. Pat. No. 5,672,662 requires expensive and time-consuming purification steps in order to provide a pharmaceutical grade product.
Thus, there remains a need in the art for improved methods for preparing polymer derivatives bearing a terminal carboxylic acid. In addition, there continues to be a need to provide novel polymers bearing a carboxylic acid moiety that are useful for conjugation reactions and further functionalization. The present invention addresses these and other needs in the art by providing, inter alia, novel methods for the efficient preparation of polymer derivatives bearing a terminal carboxylic acid.